Abstract
In susceptible plant hosts, co-evolution has favoured viral strategies to evade host defenses and utilize resources to their own benefit. The degree of manipulation of host gene expression is dependent on host-virus specificity and certain abiotic factors. In order to gain insight into global transcriptome changes for a geminivirus pathosystem, South African cassava mosaic virus [ZA:99] and Arabidopsis thaliana, 4×44K Agilent microarrays were adopted. After normalization, a log2 fold change filtering of data (p<0.05) identified 1,743 differentially expressed genes in apical leaf tissue. A significant increase in differential gene expression over time correlated with an increase in SACMV accumulation, as virus copies were 5-fold higher at 24 dpi and 6-fold higher at 36 dpi than at 14 dpi. Many altered transcripts were primarily involved in stress and defense responses, phytohormone signalling pathways, cellular transport, cell-cycle regulation, transcription, oxidation-reduction, and other metabolic processes. Only forty-one genes (2.3%) were shown to be continuously expressed across the infection period, indicating that the majority of genes were transient and unique to a particular time point during infection. A significant number of pathogen-responsive genes were suppressed during the late stages of pathogenesis, while during active systemic infection (14 to 24 dpi), there was an increase in up-regulated genes in several GO functional categories. An adaptive response was initiated to divert energy from growth-related processes to defense, leading to disruption of normal biological host processes. Similarities in cell-cycle regulation correlated between SACMV and Cabbage leaf curl virus (CaLCuV), but differences were also evident. Differences in gene expression between the two geminiviruses clearly demonstrated that, while some global transcriptome responses are generally common in plant virus infections, temporal host-specific interactions are required for successful geminivirus infection. To our knowledge this is the first geminivirus microarray study identifying global differentially expressed transcripts at 3 time points.
Highlights
In a compatible host, plant viruses manipulate and recruit host metabolites for translation and replication of their genomes and silence host responses through suppressors, despite attempts by the host to mount a defense response [1,2,3,4,5,6,7,8,9]
In a gene expression study conducted by Golem and Culver 2003 [47], a greater fold-change increase was observed in Tobacco mosaic virus (TMV) response genes in Arabidopsis Shahdara from 4 dpi to 14 dpi, suggesting that higher levels of TMV were present at a later infection time point
We propose that the higher number of induced genes at 14 dpi may reflect more of a general non-specific innate host response to virus invasion by the activation of stress and defense-like genes, whereas the increase in down-regulated genes at 24 and 36 dpi is indicative of South African cassava mosaic virus [ZA:99] (SACMV) attempt to hijack many host processes for its own benefit, leading to repression of a large number of genes
Summary
Plant viruses manipulate and recruit host metabolites for translation and replication of their genomes and silence host responses through suppressors, despite attempts by the host to mount a defense response [1,2,3,4,5,6,7,8,9]. Viral proteins are able to accumulate to much higher levels than host proteins in order to fufill their required tasks in replication, movement and suppression of host defences [4] This in turn has a huge impact on host cells and causes abnormalities in plant growth and development. Induced responses are more targeted and are triggered upon herbivorous insect or microbial pathogen attack These specific responses are co-ordinated by defense-related hormones involved in signalling pathways [3,4,10,16,17]. Abscissic acid (ABA), auxins, cytokinins, gibberellins, and brassinosteroids have been implicated [18,19] Once activated, these signalling molecules are responsible for reallocating resources away from plant growth and development towards defense. Pathogens on the other hand, are capable of manipulating these signalling networks as well as suppressing induced defenses for their own benefit, resulting in host susceptibility [16,18]
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